In WordPress you can have parent themes and child themes. Parent and child themes work together to produce your website. Not all themes have a parent/ child relationship but parent/child themes are becoming very popular and I believe will be the future of WordPress.
In a parent/child theme relationship the parent theme controls the structure of the site. The Structure being the navigation, how feature images are used, if there is a slider and many other factors. The child theme is used to make customizations of the parent them. The most common customization are changes to basic look and feel of the site. What colors are used, what fonts, images and so forth, of courses the child theme can change just but any aspect of the parent them.
Why would you use a child them. For one it makes upgrading your website easy. With a parent/child them structure you can upgrade the parent theme without changing any of your customizations.
Child themes are also a great way to practice and learn how to build WordPress sites. With a parent/child theme relationship you will already have the basic structure of the site and can modify the site. It is also easy to revert to the original look and feel of the site when you produce something you don’t like.
How do child themes work?
A child theme modifies a parent theme or framework. WordPress looks at both the child theme and the parent theme but the child them has priority. In the case of a css file the the parent css file is looked at first and the child css is looked at second. With css if two things are the same then the second one takes priority.
Lest say the parent theme has all the text in blue and you want to change the text to black. Basically all you have to do is great the child them with a new css file and set the text to black.
For find out how to create a child theme see my article how to create a child theme.